ATR

Protocol

Answer to Reset -- the initial response sent by a contact smart card after power-on, containing interface and card identification data.

Auch bekannt als: Answer to Reset

ATR

The Answer to Reset (ATRATRProtocolInitial response from card after power-on.Click to view →) is the first data a contact smart card transmits after being powered on and receiving a reset signal from the reader. Defined by ISO 7816 Part 3, the ATR encodes the card's communication capabilities, supported protocols, and identification data in a compact binary format that can be up to 33 bytes long.

ATR Byte Layout

The ATR follows a structured format starting with an initial character (TS) that establishes the bit-encoding convention:

Byte(s) Name Description
TS Initial character 3B = direct convention, 3F = inverse convention
T0 Format byte Encodes presence of TA1-TD1 and number of historical bytes
TAi, TBi, TCi, TDi Interface bytes Protocol parameters, voltage class, guard time
T1..Tk Historical bytes Card identification, OS version, chip type
TCK Check byte XOR checksum (mandatory when T=1T=1ProtocolBlock-oriented smart card protocol.Click to view → is offered)

The interface bytes define parameters such as the clock-rate conversion factor (Fi/Di) that determines baud rate, the waiting time, and which transmission protocols (T=0 or T=1) the card supports.

ATR Parsing in Practice

Developers and integrators frequently parse ATRs to identify card type and capabilities before sending APDUs. The historical bytes often contain the card vendor and product identifiers in a TLV-encoded structure, making ATR databases a practical tool for card identification. The Card Type Identifier tool on this site decodes ATR bytes into human-readable fields.

Warm Reset vs Cold Reset

A cold reset involves powering the card from scratch (VCC off then on), while a warm reset only toggles the RST line. Both trigger a fresh ATR. If the reader wants to change communication parameters, it issues a PPS (Protocol and Parameter Selection) request after the initial ATR, before sending any application-level commands.

Contactless Cards

ISO 14443ISO 14443StandardStandard for contactless smart cards.Click to view → contactless cards do not use ATR. Instead, they respond with an ATS (Answer to Select) in Type A, or an ATQB + ATTRIB sequence in Type B. The ATS carries similar information — frame sizes and protocol options — but in a format specific to ISO 14443.

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The smart card glossary is a comprehensive reference of technical terms, acronyms, and concepts used in smart card technology. It covers protocols (APDU, T=0, T=1), security (Common Criteria, EAL, HSM), hardware (SE, EEPROM, contact pad), and applications (EMV, ePassport, eSIM). It serves developers, product managers, and engineers.

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